Saggars are ceramic, box-like containers used in the firing of pottery to enclose or protect ware in kilns, or in powder calcinations for holding a powder to be calcined, or in isolation foam manufacturing for carrying the load, or in metal heat treatment. Traditionally, saggars were made primarily from fireclay. Saggars are used to protect or safeguard their load from open flame, smoke, gases and kiln debris. Modern saggars are made of various types of tailored ceramics such as for example alumina ceramics, cordierite ceramics, mullite ceramics, zirconia ceramics, magnesia ceramics, alumina-magnesia spinel ceramics, fused silica ceramics, aluminatitanate ceramics and silicon carbide ceramics.
Traditionally, saggars in commercial use used to comprise rigid rectangular boxes of unitary construction with an open top for receiving green ceramic articles placed therein for subsequent firing. Such saggars were adapted for storing vertically in the kiln for firing. Conventional saggars have a tendency to expand and contract as they are subjected to extreme temperature variations, and they often change shape, making them difficult to stack, or they may even break. It is not economically feasible to repair such saggars.
In many cases, saggars are used in heat treatments involving very rapid heating and cooling, such that high temperature gradients may appear within the saggars, leading to cracking. In the case of box-shaped solids in general, this implies a practicable upper size limit, while larger saggar sizes would be required. Furthermore, corrosion resistance at extreme temperatures is a general problem with saggars.
U.S. Pat. No. 4,008,997 discloses ceramic saggars composed of a square floor section and four identical wall sections, wherein the wall sections each comprise a flange at one end and a flange-receiving socket at an opposite end, as well as a floor supporting flange. The wall sections are assembled in positive locking engagement such that they form a square based volume and the floor section is lowered into the base of the said square. Since the base plate merely rests on the said floor supporting flanges, the assembled saggar is unstable. Furthermore, the appearance of gaps between the base and wall sections is inevitable, making this unsuitable for particulate loads, for example in powder calcinations. Despite the optional presence of gaps between the said flanges and flange-receiving sockets, which are intended to avoid the formation of thermal stresses, this does not solve the problem of upper size limits for the saggars.
The state of the art therefore constitutes a problem.